Enzymes that contain manganese are found in organisms ranging from fungi, to plants to humans. Often, manganese remains in the divalent oxidation state functioning to assist hydrolysis reactions. However, there are several cases where manganese, acting in a redox capacity, carries out chemical transformations that either have important human health consequences (e.g., the Mn superoxide dismutase, SOD) or are carrying out unique reactions with global consequences (i.e., the oxygen evolving complex, OEC). This proposal focuses on understanding the structure, physical properties and reactivity of small molecule model compounds for these manganese enzymes that contain 1 to 4 metals with oxidation levels ranging from Mn(II) to Mn(V). Specifically, we propose the following studies: Preparation of Mn complexes containing bridging imidazolates to evaluate the proposal that an imidazole may form a bridge between Mn ions in the OEC. Preparation of dimeric complexes containing Mn(V)=O and Mn(V)=N moieities to model the high oxidation states of the OEC and ammonia inhibited forms of the enzyme. Determination of Hydrogen Atom Bond Dissociation Energies for Mn complexes with coordinated water to test whether H atom abstraction is a viable mechanism for H20 oxidation. These studies will also further our understanding of oxidative reactions carried out by iron and manganese complexes. New complexes designed to function as models for new spectroscopic signatures of the OEC (e.g., recently reported epr spectra of S0 and S1 oxidation states of the OEC). Preparation of compounds with increased nitrogen and carboxylate ligation to mimic the active sites of MnSOD, catalases and arginases, enzymes which have all been structurally characterized. Upon completion of these studies, we will have a better understanding of the active site structure and mechanism of catalysis of a variety of manganese enzymes. These studies will not only answer important questions for manganese biochemistry, but should also provide information which will aid our understanding of health related, non-heme iron enzymes.